Zeta potential of microfluidic substrates: 2. Data for polymers

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Abstract
Zeta potential data are reviewed for a variety of polymeric microfluidic substrate materials. Many of these materials currently used for microchip fabrication have only recently been employed for generation of electroosmotic flow. Despite their recent history, polymeric microfluidic substrates are currently used extensively for microchip separations and other techniques, and understanding of the surface ζ potential is crucial for experimental design. This paper proposes the use of pC (the negative logarithm of the counterion concentration) as a useful normalization for the ζ potential on polymer substrates in contact with indifferent univalent counterions. Normalizing ζ by pC facilitates comparison of results from many investigators. The sparseness of available data for polymeric substrates prevents complete and rigorous justification for this normalization; however, it is consistent with double layer and adsorption theory. For buffers with indifferent univalent cations, normalization with the logarithm of the counterion concentration in general collapses data onto a single ζ/pC vs. pH curve, and (with the exception of PMMA) the repeatability of the data is quite encouraging. Normalization techniques should allow improved ability to predict ζ potential performance on microfluidic substrates and compare results observed with different parameters.